Certain types of equipment, connected to an a.c. network, generate harmonic currents. These currents may cause considerable drawbacks, for example in the form of interference in telecommunications or signal lines and increased losses in other types of equipment connected to the same a.c. network. A typical example of a type of equipment which generates harmonic currents is a convertor, for example of the type which is used in plants for power transmission with the aid of high voltage direct current. Such a convertor generates a set of harmonic currents with the frequencies EQU fn1=(n.p+1) f0 and fn2=(n.p-1)f0
where
n is 1, 2, etc. PA1 p is the pulse number of the convertor, and PA1 f0 is the fundamental frequency of the network.
A six-pulse convertor thus generates current harmonics of the ordinal numbers 5, 7, 11, 13, etc., and a twelve-pulse convertor generates harmonics of the ordinal numbers 11, 13, 23, 25, etc.
In connection with equipment for power transmission by means of high voltage direct current, it has long been well-known to connect filter equipment to the a.c. networks concerned in order to reduce the effect of the current harmonics. In typical cases, shunt filters have been used which are tuned to the lower order harmonics and possilbly, in addition, a high-pass filter which takes care of the higher order harmonics. Such filters are known, for example, from Adamson, Hingorani: "High Voltage Direct Current Power Transmission", London 1960, pp. 168-170, Adamson et al: "High Voltage Direct Current Converters & Systems", London 1965, pp. 147-162, and Uhlmann: "Power Transmission by Direct Current", Berlin-Heidelberg-New York 1975, pp. 361-376.
From the above cited Adamson et al: "High Voltage Direct Current Converters & Systems", London 1965, pp. 148, 149, 154, 155, so-called double-tuned shunt filters are previously known in this connection. Such a filter has two resonance frequencies and is normally tuned to a pair of adjacent harmonics, for example those of the ordinal numbers 5 and 7 or 11 and 13. Such a filter has the same effect as two single-tuned filters but may give economical advantages in the form of a lower installed capacitor power, lower voltage stresses on the inductors and lower power losses.
When dimensioning filters of the above-mentioned known type, the temperature dependence of the filter components must be taken into consideration. This is particularly true of the capacitances of the filter capacitors, the temperature dependence of which completely predominates over the temperature dependence of the inductances. Filter equipment of this kind is normally erected in the open and is therefore subjected to great temperature variations. The maximum relative capacitance change may typically be .+-.2%, which capacitance change gives a variation of the tuning frequency of the filter of .+-.1%. Thus, a correct tuning of the filter at a certain temperature results in a detuned filter at other temperatures. To give the filter a sufficient band width to take care of these changes in the tuning frequency of the filter, it has hitherto been necessary to design filters of this kind with a lower factor of merit than what would otherwise have been necessary. This, in turn, means that filters used up to now have had a relatively high impedance at the resonance frequency, and therefore it has been necessary to give the filters large dimensions (a high installed reactive power) to arrive at the low impedance necessary for an efficient attenuation of harmonics.
In order to avoid the drawbacks mentioned, it is known to design filters of this kind to be self-tuning. In such a filter, the inductance of the filter inductor is controllable by means of a servo motor. The motor is controlled continuously in such a way that the filter is always correctly tuned independently of temperature variations (and of variations in the mains frequency). However, it has proved that a self-tuning filter is complicated, requires a great deal of maintenance and has a poor reliability. Therefore, filters of this kind have not been used to any greater extent.
From EP-A No. 2 0 140 462, it is previously known, in circuits for filtering of electronic signals, to divide each one of the capacitors of the filter into two sub-capacitors having different signs of the temperature coefficient of the capacitance. In this way, the temperature dependence of each such capacitor, assembled from two sub-capacitors, may be reduced. From GB-A No. 720 514, it is previously known to reduce the temperature dependence of a capacitor by building an auxiliary capacitor into the capacitor, the temperature coefficient of this auxiliary capacitor having a sign opposite to that of the temperature coefficient of the main capacitor. From GB-A No. 781 763, it is previously known to reduce the temperature dependence of a capacitor by using two different dielectric materials in the capacitor.
Filter equipment of the kind referred to here typically has very large dimensions, and dividing each capacitor into two sub-capacitors, or specially constructing each capacitor with an auxiliary capacitor or with two different dielectrics, would entail considerable drawbacks, for example in the form of a considerable increase of the cost of the equipment.